Optically pumped magnetometers (OPMs), also called atomic magnetometers, optical magnetometers, or optical atomic magnetometers, operate by measuring the transition frequency between two magnetically sensitive ground states of atoms with unpaired electrons such as rubidium (Rb), cesium (Cs), and potassium (K) for example. For an overview, see Budker and Romalis (Budker and Romalis, 2010). When measuring the transition frequency between Zeeman levels of the same hyperfine ground state, the frequency is in the kHz range, and when measuring the transition frequency between Zeeman levels of two different hyperfine states, the frequency is in the GHz range. Building a magnetometer using Zeeman levels of two different hyperfine states is generally less preferred due to the need for additional microwave circuitry, and because the signal-to-noise ratio is often found to be lower. For this reason, all commercial variants of the optically pumped magnetometers available today rely on Zeeman transitions within the same hyperfine state. Here we disclose a device that measures magnetic field gradient, a gradiometer, in place of magnetic field, magnetometer, and in this case we find the use of Zeeman levels of separate hyperfine transitions to be particularly advantageous in addressing the drawbacks of hyperfine magnetometers described in prior-art, as in Arditi, U.S. Pat. No. 3,281,663A, and Alzetta, 1976.
For many applications, such as brain imaging, magnetic gradiometers, devices that measure the difference in the magnetic field at two locations, are greatly preferred over pure magnetometers due to gradiometer's ability to remove common mode magnetic field noise. However, building intrinsic gradiometers, devices that measure magnetic gradient field directly, has proven difficult without first building two separate magnetometers and then subtracting their outputs, as in a synthetic gradiometer. The synthetic gradiometer approach is technologically more complex and does not provide perfect cancellation over all frequency range. In the prior art, potential gradiometer schemes using hyperfine transitions have been disclosed (Affolderbach, 2002), but they are synthetic and not intrinsic. Here we disclose a system and a method of building a very high-performance intrinsic gradiometer leveraging hyperfine transitions.